One type of marine diesel propulsion engine is characterized as a slow speed, two-stroke engine which is frequently referred to as a cross-head engine. In a cross-head engine the firing cylinder and the crankcase are lubricated separately by a cylinder oil and a marine system oil, respectively. A marine system oil lubricates the crankcase of a cross-head marine diesel engine and may cool parts of the engine, especially the undercrown of the pistons. In some engines, the marine system oil also performs the functions of lubricating gears and fuel pumps. Properties that are typically important for systems oils are oxidative stability, viscosity increase control and the detergency of the oil.
The development of ever more sophisticated engines results in an increasing sensitivity to fuel contamination of the lubricating oil, which can lead to undercrown deposit formation in the pistons. As a result, requirements for marine system oils for two-stroke engines become ever more stringent and demanding. Recently new “carboxylate” detergents have been developed, as described in patent application publication U.S. 2007/0027043. The inventors have discovered that such detergent additives, which comprise an overbased salt of at least one alkyl-substituted hydroxyaromatic carboxylic acid wherein at least 50 mole % of alkyl groups are C20 or greater, may be used in marine system oils, and that the resultant marine system oils provide superior performance in at least the following areas of oxidation stability, viscosity increase control and detergency than the currently available technology, especially when compared to conventional salicylate-containing detergent technology.
There are numerous overbased detergents and other additives for use in lubricating oils, and the following documents present a few:
U.S. Patent Publication No. 2007/0027043, the entirety of which is hereby incorporated by reference, describes the production of over-based detergents formed by over-basing alkaline earth metal alkylhydroxybenzoates or a mixture of one or more of these with alkylphenols.
U.S. Pat. No. 7,163,911, the entirety of which is hereby incorporated by reference, describes an unsulfurized, single-ring carboxylate-containing additive produced by neutralizing hydrocarbyl phenols using an alkaline earth base in the presence of a promoter, carboxylating the hydrocarbyl phenates using carbon dioxide under carboxylation conditions sufficient to convert at least 20 mole % of the starting hydrocarbyl phenols to hydrocarbyl hydroxybenzoates, and separating at least 10% of the starting hydrocarbyl phenols from the formed product.
U.S. Pat. No. 5,895,777 describes lubricating oil additives comprising the alkaline-earth metal salts of aromatic carboxylic hydroxy acids containing carboxylic acids having 16 to 36 carbon atoms.
European Patent Application No. 1,154,012 describes lubricating compositions comprising an oil, an anti-wear additive and a sole oil-soluble overbased detergent comprising an aromatic carboxylate, such as a calcium salicylate substituted by a hydrocarbon remainder.
British Patent No. 1,146,925 describes lubricating compositions comprising, as lubricating agents, polyvalent metal salts, in particular calcium, and alkylsalicylic acids comprising more than 12, preferably 14 to 18 carbon atoms in the alkyl group. These salts can be prepared from the corresponding sodium salts, as synthesis intermediates.
British Patent No. 786,167 describes polyvalent metal salts of oil-soluble organic acids, such as sulfonic hydrocarbons, naphthenic acids or alkylhydroxybenzoic acids, in particular alkylsalicylic acids having an alkyl radical of up to 22 carbon atoms. The alkylsalicylic acids can be prepared from sodium alkylsalicylic acids according to the processes described in British Patent Nos. 734,598, 734,622 and 738,359. The sodium alkylsalicylates described in these British patents are described as being useful as synthetic intermediates for the preparation of alkaline-earth alkylsalicylates, and useful as additives for lubricating oil.
European Patent Application No. 1,229,101 describes a two-stroke cross-head marine compression-ignited system lubricant, such as diesel engine system lubricant, obtained by admixing a base stock and an oil-soluble overbased metal detergent additive, in a minor amount, in the form of a complex.
European Patent Application No. 1,229,102: A lubricating oil composition comprises an admixture containing: (A) primarily a base stock of lubricating viscosity (base oil); and (B) a minor amount of an oil-soluble over-based metal detergent additive in the form of a complex in which the base material is stabilized by more than one surfactant.
European Patent Application No. 1,728,849 describes a lubrication for a cylinder liner and crankcase in marine diesel crosshead engine that comprises greater than or equal to 40 mass % of an oil of lubricating viscosity, at least one detergent, at least one dispersant, and at least one anti-wear additive.
European Patent Application No. 1,778,824 describes a lubrication for internal combustion engines that comprises selecting an additive package to provide a desired total base number level to the lubricating composition to modify performance characteristics of the engine. The additive package having a viscosity of 2-12 mm2/s comprising light neutral base oil to form a lubricating composition.
In general, the above references describe overbased detergents and other additives for use in lubricating oils that are not taught for use in the environs where marine system oils are required. Thus, it is desirable to have a marine system oil composition suitable for use, for example, in the crankcase of a slow-speed, two-stroke cross-head marine diesel engine.